For a number of years, tire manufacturers have been devoting significant effort to developing novel solutions to a problem that dates back to the very start of the use of wheels fitted with tires of the inflated type, i.e. how to allow the vehicle to continue on its journey in spite of a substantial or complete loss of pressure from one or more tires. For decades, the spare wheel was considered to be the only and universal solution.
Then, more recently, the considerable advantages associated with omitting the spare wheel appeared. The idea of “extended mobility” was developed. The associated techniques allow a vehicle to continue to drive along without removing the tire, even after a puncture or significant loss of pressure, provided that certain instructions are observed. This for example means that a repair facility can be reached without the need to stop, under circumstances that are often dangerous, in order to fit the spare wheel.
Today, there are two major types of run-flat technology. On the one hand, there are wheels equipped with supports, capable of supporting the inside of the tread of a tire when the sidewalls collapse following a drop in pressure. This solution is advantageously combined with a tire comprising a bottom region capable of minimizing the risks of the tire slipping off the rim. This solution has the advantage that the tire performance under normal conditions is very close to the performance of a conventional tire. By contrast, it does have the disadvantage of entailing the use of an additional component, namely the support, for each of the wheels of the vehicle.
On the other hand, there are tires of the self-supporting type (often known by their English-language acronym “ZP” which stands for “zero pressure”). These self-supporting tires are capable of bearing a significant load at a reduced pressure, or even without pressure, because they have sidewalls which are reinforced, usually by means of rubber inserts provided in the sidewalls. For the sake of simplicity, these tires will hereinafter be referred to as “run-flat tires”.
Tires such as this are known, for example, from U.S. Pat. No. 4,067,347, U.S. Pat. No. 4,779,658, U.S. Pat. No. 5,511,599, U.S. Pat. No. 5,769,980, U.S. Pat. No. 5,795,416, U.S. Pat. No. 6,022,434 and U.S. Pat. No. 7,093,633.
While these tires offer satisfactory service under run-flat conditions, they have the disadvantage, under normal running (that is to say when inflated to their service pressure or pressure close to this service pressure), of having inferior performance compared to conventional tires, particularly as regards ride comfort and rolling resistance. The drop in comfort can be explained by the fact that the additional rigidity afforded by the reinforced sidewalls penalizes the rolling flexibility of the tire, making driving unpleasant. The increased rolling resistance, for its part, stems from the addition of material to the sidewalls; these materials, because of their deformation and hysteretic losses, contribute to increasing the resistance of the tire to rolling.